Liquefied natural gas is commonly referred to by the acronym ‘LNG’. During recent years LNG has become an increasingly more sought-after energy resource. It is expected that natural gas will to an ever greater degree replace oil as an energy source.
It is known to cool natural gas down to about −163° C. to produce LNG at dedicated onshore export terminals. It is also known to load LNG into purpose built LNG tankers to transport the LNG at approximately atmospheric pressure to dedicated receiving terminals around the globe. It has been proposed for some time, that floating offshore structures, such as floating liquefaction vessels (referred to in the art as ‘FLNG vessels’), could be used to liquefy natural gas although no such vessel has been put into production at this time.
It has been proposed that an FLNG vessel will be permanently moored to the seabed at a desired production location using a ‘spread mooring system’. A spread mooring system relies on attaching heavy mooring lines or chains to the hull of the FLNG vessel and anchoring the chains to the seabed to ensure that weathervaning cannot occur. However, a spread mooring system is only an option in relatively benign locations where the prevailing weather is known to be highly directional. Such locations are not common.
Alternatively, it has been proposed that an FLNG vessel will be permanently moored to the seabed at a desired production location using a single point mooring system connecting it to the seafloor via a series of mooring lines (typically chains or wires). The mooring lines extend below sea level to the ocean floor and can cost in the order of one hundred million US dollars. A single point mooring system is placed within or adjacent to the FLNG vessel. The single point mooring system is designed to receive a stream of hydrocarbons delivered to the single point mooring through one or more production risers connected to wells on the sea floor. In addition to this, well risers, umbilicals and other subsea services necessary to the operation of the FLNG vessel and its associated feed gas architecture pass through the single point mooring system. In addition to performing this function, prior art single point mooring systems are designed and sized to moor the FLNG vessel at or near a preset longitude and latitude whilst allowing the FLNG vessel to freely weathervane around the single point mooring. Such single point mooring turrets are designed and sized such that the FLNG vessel can remain moored and weathervane around the single point mooring system whilst withstanding the forces of up to a 10000 year storm so that FLNG vessel remains fixed to the single point mooring at all times during the producing life of the FLNG vessel. Consequently, the proposed FLNG vessel are designed to have no means for self-propulsion with the result that it operates more like a barge than a ship.
Using the single mooring systems currently proposed for use for FLNG vessels, the proposed FLNG vessel is held on a station keeping point by the suitably sized single point mooring system and the orientation or ‘heading’ of the FLNG vessel is primarily dependent on the weather conditions, current direction, wind direction, and wave direction. Such single point mooring systems are extremely large, extremely complex and extremely expensive, costing in the order of 500 to 900 million US dollars. If there is a desire to hold the FLNG vessel at a heading that differs from the weathervaning heading, the FLNG vessel must be fitted with thrusters that are located aft of the single point mooring system so as to cause the FLNG vessel to be rotated around the single point mooring system, either alone or in combination with a separate self-propelled vessel such as a tug boat that is used to apply a local pushing or pulling force to the hull of the FLNG vessel to provide heading control.
There remains a need for an alternative system for heading control of an FLNG vessel during offloading of LNG.